The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display using a liquid crystal having a spontaneous polarization.
Along with the recent developments of the so-called office automation (OA), OA apparatuses, typically exemplified by word-processors and personal computers, have been widely used. As such OA apparatuses have become prevalent in offices, there have been ever-increasing demands for portable-type OA apparatuses that can be used in offices as well as outdoors; and there have been also demands for small-size and light-weight of such apparatuses. Here, liquid crystal displays have come to be widely used as one of the means to achieve such an objective. Liquid crystal displays not only achieve small-size and light-weight, but also include an indispensable technique in an attempt to achieve low power consumption in portable OA apparatuses that are driven by batteries.
The liquid crystal displays are mainly classified into the reflection-type and the transmission-type. In the reflection-type liquid crystal displays, light rays that have been made incident on the front face of a liquid crystal panel are reflected by the back face of the liquid crystal panel so that an image is visualized by the reflected light. In the transmission-type liquid crystal displays, transmitted light from a light source (back-light) placed behind the back face of a liquid crystal panel is used to visualize an image. Although those of the reflection-type are inferior in visibility due to irregularity in the amount of reflected light that depends on environment conditions, they are inexpensive and widely used as display devices with mono-color (for example, black/white display, etc.) for such as calculators and watches. However, they are not suitable for display devices for personal computers, etc. which carry out a multi-color or full-color display. For this reason, in general, transmission-type liquid crystal displays are used as display devices for personal computers, etc. which carry out a multi-color or full-color display.
Here, currently-used color liquid crystal displays are generally classified into the STN (Super Twisted Nematic) type and the TFT-TN (Thin Film Transistor-Twisted Nematic) type based upon the liquid crystal type to be used. Although those of the STN type have comparatively low manufacturing costs, they are susceptible to cross-talk, and comparatively slow in response speeds; therefore, they are not suitable for display for animation pictures. In contrast, those of the TFT-TN type have higher display quality as compared with the STN type; however, since, at present, their liquid crystal panel has a light transmittance as low as 4%, a back-light with high luminance is required. For this reason, those of the TFT-TN type have greater power consumption due to the back-light, resulting in a problem in use of carrying battery power-source. Moreover, the TFT-TN type have other problems with the response speed, particularly slow in response speed for displaying half tones, narrow viewing angle, difficulty in adjusting the color balance, etc.
In order to solve the above-mentioned problems, a new type of liquid crystal devices have been developed in which a liquid crystal (ferroelectric liquid crystal or anti-ferroelectric liquid crystal, etc.) having a spontaneous polarization, which has a high response speed of hundreds to several μ seconds order to an applied electric field, is used and this liquid crystal is driven by using a switching element such as a TFT. In the case when such a ferroelectric liquid crystal or an anti-ferroelectric liquid crystal is used, the liquid crystal molecules are constantly maintained in parallel with the substrate (glass substrate) independent of the presence or absence of an applied voltage so that it is possible to achieve a very wide viewing angle.
However, in the liquid crystal display using a liquid crystal having a spontaneous polarization, such as a ferroelectric liquid crystal or an anti-ferroelectric liquid crystal, that is driven by using a switching element, there is a problem in which the driving operation requires a greater voltage.
FIG. 1 is a graph that shows the relationship between the applied voltage and the light transmittance in a conventional liquid crystal display using a ferroelectric liquid crystal. FIG. 1 shows that an applied voltage of ±15 V is required to adjust the light transmittance of the liquid crystal; however, from the viewpoint of voltage resistance, etc., of the liquid crystal driving driver IC, etc., it is preferable to set the applied voltage to not more than ±10 V.